Myth(?) of the Jackrabbit Start

[MCR]

Originally Posted by Lug_Nut

Foot to the floor means there is no more reserve of power available. The engine, at that rpm, IS at 'full load' and at maximum efficiency of converting fuel into energy.

I'm having trouble understanding how max efficiency can be at full load. Are you saying that at any given rpm, applying full throttle allows the engine to be at maximum efficiency? That is not making sense to me. Isn't the engine struggling to accelerate? Wouldn't it be trying to overfuel in order to gain rpm?

In a typical engineering problem, there are many different efficiency calculations possible. I could probably think of a dozen efficiency terms to describe an electric motor running an air compressor. Some of them don't mean a whole lot---For example "volumetric efficiency" is just a ratio of pressures. If you compress from 50 to 100 psi, the volumetric efficiency is 1/2. Big whoop.

It just doesn't make sense to me that max efficiency would be a line of 100% load request. The max efficiency should be a point or a small range of points (a hilltop of a contour map) of a specific rpm and engine state (I'm not sure what I mean by engine state, but I'm thinking boost and other settings of controllable items, such as EGR. It probably changes with weather and stuff like altitude.)

For questions like this, I like to see a clever experiment.

Is it possible to make a diesel purge like apparatus that would let you make accurate measurement of fuel usage during some acceleration tests?

When I was a kid I had this piece of chemistry labware for titrating. (My mom used it at work or something). It had a squeeze bottle at the bottom with a glass tower. When you squeezed the bottle, it overfilled the glass tower---when you let go, the fluid drained back into the squeeze bottle, but since the drain was consistent, you always started a titration with the same amount of fluid. The glass tower was graduated, and it had a stopcock at the bottom. (After some searching---it's a self-zeroing burette: see http://www.biodieselcommunity.org/titrationstation/ for an example.)

It might be possible to rig up something (perhaps not as intricate as the glass tower thing) so that you could accurately measure a variety of starts. I'm fairly certain there might be a suitable electronic fuel flow gauge available, but the burette thing was the first thing that popped into my head.

One problem is how you choose the test run. You can't stop at a certain speed, because the faster accelerating cars would be charged less distance. You can't allow infinitely slow acceleration because that would never happen in real traffic and it would make your test road very long. You'd probably have to design the test around your road (or airstrip or whatever), and then accelerate to a speed and then hold it for the test section. That way a slow accel could be compared over the same distance as a hard accel.

Another problem is handling the recirc of an ALH, and it's propensity for air bubbles that might skew the results. I don't know much about other engines.

[ejpilger]

This is exactly what led to my original question. Along with the excellent point about the "psychological" aspects of jack rabbit starts it pretty much validates what I was thinking. The only thing I am left wondering is whether it would also be true for gassers.

As for questions about ScanGuage vs. Fillups; although my recent comments have been based on the SG, the results have been validated by past fills. The last time I had the car all to myself and drove efficiently, under cruise control, I barely made 700 miles. This time, with P&G, I appear to be on track for 800 miles.

As for P&G, it clearly works. Over the last two day, I took identical freeway runs, both ways, one with P&G between 50 and 60, and one with CC at a constant 55. The CC averaged 54 MPG, while the P&G averaged 62.

Originally Posted by Lug_Nut

Back to 'jack rabbit'?
Foot to the floor means there is no more reserve of power available. The engine, at that rpm, IS at 'full load' and at maximum efficiency of converting fuel into energy.
Converting that energy into distance with maximum efficiency is not so simple as mashing the accelerator to the floor.
A direct coupling is needed, no automatic torque converter, no clutch input/output shaft speed mis-match (slip), no tire to pavement speed differential (wheel spin) and no WOT acceleration in a gear lower than needed.
Start rolling at engine idle in first gear. Only put the pedal to the floor once the clutch is fully engaged.
Regulate the accelerator. First gear will frequently be low enough that WOT causes wheel spin. Avoid wheel spin by backing off the go pedal. 80% load and no slip is more efficient than 100% load with clutch slip or wheel spin.
Select second gear as soon as the vehicle speed is high enough that the engine will be at idle rpm or higher. Don't ride the clutch since the engine won't stall. Use WOT. The gearing is such that the chance of wheel spin is less than in first so accelerator pedal modulation is less likely to be needed to avoid wheel spin. Second is used only because of this reduced wheel spin advantage versus staying in first. Continue accelerating at WOT until the desired speed is reached, or until the fuel consumption curve shows that another gear ratio with the engine at full load will have lower consumption. That typically happens at about 2500~2700 rpm in the lower gear and 1500~1700 in the next higher one. In other words: If full load at 1600 rpm in third is as efficient as 2600 rpm in 2nd, shift out of 2nd at 2600 and into 3rd.

If the desired speed is reached before running out of gears, then shift into the highest gear that can maintain that vehicle speed with the engine at as slow an RPM as possible, that may actually mean accelerating as hard as possible in 3rd to 30 mph and then straight into 5th at 1100 rpm if there is no need to go faster than 30.
This technique is the most efficient means of building vehicular momentum by getting into the highest gear ratio at the lowest engine rpm with the lowest consumption of fuel. It works in TDI diesels. It may not be applicable to throttle plate restricted air intake engine.
It also works only when that momentum can be maintained. It is idiocy to use it in stop and go traffic or where there are timed red lights at each intersection.
Acceleration is a waste of fuel. Accelerating again is a greater waste because brakes have also been squandered in between.

[ejpilger]

Originally Posted by syrjohn

ejp,

Are those mpg figures from your SGII or from fillup? I find my readings can vary quite a bit while my fillup calculations are much closer.

jep

ScanGuage Though I have never seen quite the inconsistencies that you have.

Happy belated (or premature) fathers day.

[MikeMars]

Originally Posted by ejpilger

...
As for P&G, it clearly works. Over the last two day, I took identical freeway runs, both ways, one with P&G between 50 and 60, and one with CC at a constant 55. The CC averaged 54 MPG, while the P&G averaged 62.

This experience matches mine. But since this thread has covered both diesel and petrol engines, we should remember that petrol engines will not do nearly as well with P&G if the engine is left running (since their idle is very inefficient compared with diesel). I gather that the equivalent technique is actually to stop the engine during the glide period (I've experimented briefly with this, and gave it up as hazardous to life and limb...)

[Zero10]

This argument has been had several times on here, and I have even been on the wrong side of the fence a time or two. From a mathematical perspective, each acceleration or trip comes down to a simple traversal of the engine efficiency graph, but that graph must also be plotted against time before you can do this (you need an extra axis on this graph, it must be 3-dimensional before you traverse it since there is no time axis on the graph already). Once you take into account the area under your curve when the existing graph is plotted against time, you will have your net fuel consumption for the trip. I wrote a program to do some calculations on this new graph and every time it comes out in favor of accelerating "reasonably" slow. NOTE: It does not favor accelerating as slow as humanly possible, once you reach a certain point it becomes unfavorable to accelerate any slower. I long ago lost this program but if my memory serves me it is roughly when it takes 30 seconds to reach 60mph assuming reasonably constant acceleration. When I compared the results to my driving style I realized that for maximum MPG I would need to obstruct traffic and waste time, and I'd rather burn a touch more fuel than get run off the road by an angry person.

Basically, if you do a quick start yes you start out very high efficiency but you are also spending this "more efficient" time burning more fuel. I don't want to start any wars or arguments, but when I worked this out it also favored constant throttle input over P&G assuming road conditions stay constant. I'm not the smartest guy in the world and I had already spent too much time on it, but I couldn't work out a clean way to incorporate road conditions (slope, wind, etc.) into the calculations in a meaningful way so I can't really speak to that. I also didn't consider stopping the engine for pulse and glide, since many of the intervals are so short that it just isn't practical.

Long story short, slower constant acceleration is best with steady throttle input on level ground, efficiency does not always equal economy, and stopping the engine on the highway can be very dangerous.

[tasdrouille]

For every engine, there is an optimal acceleration operating line that can be traced on the bsfc map. That is the line that would get you the best FE under acceleration if you could follow it perfectly (CVT comes in handy here). This line can generally (depending on the engine) be found near the 85% load range, but it sure is never around the 25% load range.

Get to your target speed quickly, then you can feather the go pedal and use the highest gear possible. That's what gonna give you the best FE without too much hassle/concentration on driving.

[Lug_Nut]

To think what a fool I was using my combination technique of 'pulse&maintain' to average 82 mpg over 250 miles mostly highway with a stop & go access in the 2003 Tour de Sol. Or that my car returned an average of 77 mpg over 500 miles of suburban / rural driving in the 2005 Tour de Sol.
I know what works for me.

[JyRO]

Originally Posted by Zero10

...Long story short, slower constant acceleration is best with steady throttle input on level ground, efficiency does not always equal economy, and stopping the engine on the highway can be very dangerous.

OK, this guy makes perfect sense. He is right. It's the area under the curve. Except P&G when done right does work, and his analysis why straight and level is more fuel efficient just needs to be studied why it does not agree.

Those of you who think flooring it to your speed and skipping gears is MORE FUEL EFFICIENT, need to come back to reality. If that were true, NASCAR vehicles would be very efficient. If that were true, hooking large parachutes to your cars so you would keep a very high load and running at whatever high rpm "the graph" says is peak efficiency would return your best mpg. It's just so much nonsense I won't waste any more time on it.

rotarykid - Your driving tactics and your last post are right on.

njkayaker - X miles over 0 fuel consumption IS infinite mpg.

Originally Posted by njkayaker

Momentum is energy. Since you don't go as far using your brakes than you would coasting, you have to add extra fuel to make up the loss. Thus, clearly, braking is less efficient and uses more fuel than not braking.


Quote:
Originally Posted by JyRO
That's why I'm saying, braking does not burn fuel. And wasting momentum does not use fuel ... returning to a set speed, or gaining speed that was lost is what uses fuel.

Braking does use fuel. Wasting momentum uses fuel. Imagine how much more fuel you would use stepping on the brake while you are stepping on the thottle. Since momentum only comes from fuel, wasting it has to waste fuel. Think of momentum as exactly like fuel.

This whole discussion is about mpg. And we use diesel fuel or gasoline for the most part to power our cars. Braking does not waste fuel. Using fuel wastes fuel.

Seriously - humor me & read the below carefully, and lean back in your chair and think about it. I'm not trying to be sarcastic as if you cannot understand, I'm seriously requesting you think deeply about the sitution and the meaning of the words below:

If a person speeds up approaching a RED traffic light, and then has to use the brakes to slow down, or slow to a stop, then the wasting of fuel took place when the person used fuel to ACCELERATE unnecessarily. The brakes wasted the potential energy that they had built ... but we don't pull up to a fuel pump and put potential energy into our tanks ... WE PUMP FUEL. Yes, fuel is energy, it's used to create potential energy (momentum) and we'd be more fuel efficient NOT to waste the potential energy provided by the fuel. It's when the fuel is used and how the fuel is used that determines fuel efficiency. We both agree that using brakes contributes to lower fuel efficiency. But brakes use NO FUEL.

So anyway, we're all just chasing our tails on this issue. Some want to believe that flooring it is fuel efficient (and they need
counseling ) ... and njkayaker and I are arguing over which came first, the chicken or the egg. Basically njkayaker and my driving style would be exactly the same. So I don't see any point to responding to this thread any longer ... it's unnecessarily increasing my blood pressure. Good luck fellers. I'm satisfied with 40 mpg in my Corolla, gently accelerating, no aggrevating P&G driving style and no going below the speed limit and pissing people off.

If people get pissed off at me for observing the speed limit, then it's on them.

[ejpilger]

Although it steps outside the bounds of my original question, the idea of towing parachutes does introduce an interesting additional point. It probably IS more efficient, if what you are measuting is MPG/lb.

In other words, since the TDI is most efficient at turning fuel into force at high loads, then if you were paying for each pound transferred, you would want to load it up (Though this seems like a no brainer, it wouldn't be true if the efficiency went all to hell at high loads.) HOWEVER, I was playing the game we are all mostly playing, and trying to get the best out of a car with only me in it and the AC off. This means I need to find other ways to increase my load.

BTW, in a great stroke of inspiration, I now see that MPG/lb is the same type of unit as ft-lbs/G, or torque achieved per fuel used, which is what I am trying to optimize (without, of course, wasting it by slipping wheels, clutch, etc.)

For me, it's all about converting a given amount of fuel energy into the maximum amount of kinetic energy, and the minimum of other types (heat from friction with brakes, road, clutch, air), under standard conditions. Of course, not at the expense of driving people to road rage behind me, or being pulled over for my erratic driving

[njkayaker]

Originally Posted by JyRO

njkayaker - X miles over 0 fuel consumption IS infinite mpg.

x/0 is not concidered to have any real meaning in mathematics.

Originally Posted by JyRO

Braking does not waste fuel. Using fuel wastes fuel.

Braking wastes fuel the same way as driving with your windows down wastes fuel or carrying unnecessary weight wastes fuel. Braking wastes fuel the same way as leaving a rechargable flashlight on wastes whatever fuel was used to generate the electricity used to recharge the battery.

Originally Posted by JyRO

If a person speeds up approaching a RED traffic light, and then has to use the brakes to slow down, or slow to a stop, then the wasting of fuel took place when the person used fuel to ACCELERATE unnecessarily.

Yes, if the person could knew that the red light was going to change then he wasting fuel at that point. Clearly, the whole idea is to never gain momentum that you have to waste by the act of braking (ie, anticipation). And if you could anticipate everything 100%, you would not need brakes.

Once you have that momentum (and are no longer burning fuel), you can't go back in time to reduce the fuel consumption that produced it. You can only conserve or consume the momentum. The act of braking consumes energy. Therefore, the act of braking indirectly (obviously) consumes fuel because the energy consumed came from the fuel.

Originally Posted by JyRO

but we don't pull up to a fuel pump and put potential energy into our tanks

Hybrids do exactly that. Hybrids "pull up to the fuel pump" every time they brake.

[njkayaker]

Originally Posted by JyRO

If a person speeds up approaching a RED traffic light, and then has to use the brakes to slow down, or slow to a stop, then the wasting of fuel took place when the person used fuel to ACCELERATE unnecessarily.

Well, no, the wasting of fuel occurs with the choice to slow down. That is, it would be more efficient to run the red light.

There are many times where braking is a choice. If you are on a highway with a car ahead of you, you can often choose to brake or to move into an empty lane. The choice to brake is the more fuel inefficient. It's the act of stepping on the brake that wastes the fuel.

The wasting of the fuel did not occur at acceleration because the person could have chosen not to brake!

Momentum is just a way of storing energy. It's a mechanical battery. The purpose of burning fuel is to create momentum. Anything that decreases momentum wastes fuel (because all of the energy comes from fuel).

Hybrids prove that braking wastes fuel. Because one of the purposes of a hybrid is to recover the energy (obtained from burning fuel) wasted as heat by ordinary brakes.

If the goal of shooting a gun is to hit a target, It's a waste of gun powder if the bullet misses the target.

Of course, one major reason highway driving is more efficient is because highways reduce the need to brake.

[Lug_Nut]

The post is about "jackrabbit starts", not braking.

Operating at full load, the very top of the chart curve, up to the speed desired and then holding the load needed to maintain that speed IS more fuel efficient at turning fuel into distance than a lesser acceleration rate using a lower portion of the engines capacity.
Example A: Acceleration in 5th gear from 35 mph to 70 mph at WOT or full load will use some amount of fuel at each point along the very top edge of the curve as the car accelerates. At 70 mph the accelerator is relaxed to the fueling request that is needed to maintain speed against the drag at 70 mph.
Example B: Acceleration in 5th gear from 35mph to 70 mph, but at half load. This will use less fuel because only half the power capacity is being requested. This fuel use curve during acceleration will be 1/2 way vertically between the zero load fuel use and example A's top of the graph. At 70 mph the accelerator is relaxed and held as in example A. From that point onward example A and example B are identical.
What appears to be overlooked by most that claim this lesser acceleration rate is more fuel efficient is that the acceleration rate of the 1/2 load operation has taken twice the time to achieve 70 mph. For each instant that the full load operation is at some RPM and fuel use, the half load acceleration spends twice the time at that rpm, BUT THE FUEL USE IS MORE THAN HALF! Doubling the time of the acceleration but not cutting fuel in half is going to, and will always, use more fuel.
In addition, because example A reached the desired speed sooner, there is less time at lower speeds making a faster average speed.

Pulse & Glide doesn't work either.
P&G between 45 to 55 mph. The drag load from 45 to 50 is less than from 50 to 55. Raising the speed 10% from 50 will increase the air drag 21% (50=1, 55=1.1, 1.1x1.1=1.21). It will take longer time (presuming constant power output from the engine) to accelerate against the greater drag from 50 to 55 than it takes that same constant power to accelerate from 45 to 50. More time use at this higher drag load from 50 to 55 isn't counteracted completely by the lower drag between 45 and 50. Drag at 45 mph is 81% (50=1, 45=.9, .9x.9=.81). Air drag at 55 is 21% higher, but air drag at 45 is only 19% lower. You've lost 2% of any fuel gains already.
You'll coast from 55 to 50 over a less distance, and for less time, than you'll coast from 50 to 45. The higher drag at higher speed will slow the car more than the lower drag at the lower speed. The result is your average speed will be less than 50.
Wait! More fuel and lower average speed? Yep, sorry for bringing reality to light.
Increasing the the P&G speed range (40 to 60 mph, for example) increases the P&G loss. Drag at 60 is 44% greater than at 50, but only 36% less at 40, an 8% net increase in drag. The greater the P&G range, the worse the total net increase in air drag, and the greater the fuel consumption becomes. Reducing the range of speed diminishes this effect. Reducing the range to zero is the only effective P&G range, but then it isn't P&G, is it?
The use of full load (or of any increase in load) does turn fuel into power more efficiently, but when the increased power is being used to push ever harder against air your added efficiency is merely adding dust to the wind you are creating. It does not turn fuel into distance more efficiently.
I'm done.

[rotarykid]

The part of your argument that doesn't really apply to the real world is no where I drive is 100 % flat or is always up or down a grade but a mix of all these conditions . A small pulse up to close to desired speed , low speeds over a stretched out slight down grade is more efficient than a steady speed staying in gear . If you can hold enough speed/energy over this glide you can over come the slowing effect of a slight up grade thrown in without extra energy/fuel input .

I have a 3-4 mile stretch of 30-40 mph road between me & the freeway in Denver where a small amount of energy input gets me almost all the way to the end if I use the terrain . According to the scan gage I'm in the 60-70 mpg range at the end which helps the calculations for the rest of the day if I do this pulse & glide on my low speed stretches . If I don't and just drive in gear the readings are in the 30s to low 40s mpgUS on the scan gage for the day .

Where I live in eastern NC there are also places where I can use the terrain to increase my low speed mpgs to extreme amounts .

I also verify these numbers with pen & paper every tank . If I put the extra effort in I always achieve above the 52-55 mpgUS mark in my city loops . If I don't put any extra effort it is from 48-52 mpgUS . I have 8 years of data for this route with these mpg numbers .

[EddyKilowatt]

Originally Posted by Lug_Nut

Operating at full load, the very top of the chart curve, up to the speed desired and then holding the load needed to maintain that speed IS more fuel efficient at turning fuel into distance than a lesser acceleration rate using a lower portion of the engines capacity.

I am not going to join the back-and-forth here, but every time I have thought this through, I have come to the same conclusion (though I might suggest "nearly full" rather than "full" engine load).

The key point is the idea of turning Fuel into Distance. (with some secondary considerations like Getting There Today, and Not Being A Rolling Traffic Obstacle, and What Is The Effect Of Non-Steady-State Operation On The Engine BSFC Map).

Though I think that the thermodynamics and physics are pretty straightforward, we should all be open to some real-world testing. I submit that getting Heavy Right Foot and Velvet Right Foot exponents together, driving a fixed course in a fixed time, then swapping cars and repeating the drive, would be an interesting way to shed some light on this question.

Eddy

[MikeMars]

Originally Posted by Lug_Nut

...
Wait! More fuel and lower average speed? Yep, sorry for bringing reality to light.
...

In practice (not in theory) I experience slighly less fuel and slighly higher average speed when using P&G. Note the use of 'slightly'... (a few %) :-) I have driven for many tanks at a constant of 50 mph on the motorway, and I have driven for 3 tanks using a P&G of 50 to 60 mph in the same situation.

Your analysis is mainly looking at air resistance (which is indeed an important factor at speeds over 60 mph). But engine resistance is another factor which I personally feel is the dominant feature of P&G. I find that the glide is usually much longer than the pulse, for example typically I may pulse for 10 seconds at 35mpg (imp) instantaneous, and then glide for 40 seconds at 0.2 gallons (us) per hour.

In theory I would have expected a stronger throttle to give better results judging from the graphs, but practical experimentation seems to point the other way.